1. Field of the Invention
The present invention relates to methods and apparatus for printing which compensate for a variable distance between a printhead and a recording medium.
2. Description of the Related Art
A standard printer architecture for low volume products employs a printhead on a moving carriage, printing on paper which conforms to a cylindrical platen or roller. A certain class of these printers, such as some thermal ink jet printers, use a printhead having a line of printing elements which is perpendicular to the axis of the curved platen. As a result, some of the printing elements are farther away from the paper than others. By positioning the printhead such that its central element is closest to the paper, the overall distance difference is minimized. FIG. 1 shows a side view of a printhead centered near a curved platen. At the center of the printhead the distance to the platen is D, but in general is z=D+d. If r is the radius of the platen and y is the distance above or below the printhead center, then d=r(1-(1-(y.sup.2 /r.sup.2)).sup.0.5). If y is much less than r, d is approximately y.sup.2 /2r.
Because the carriage is moving at velocity v.sub.c and the nozzles are not at uniform spacing from the paper, there will be a spot placement error in the x direction (the direction of movement of the carriage containing the printhead) such that .DELTA.X.sub.z =.DELTA.z(v.sub.c /v.sub.d) where v.sub.d is the drop velocity and .DELTA.z is the difference in distance from the platen between the furthest nozzle and the closest nozzle. For a curved platen, where .DELTA.z=d and d approximates (.congruent.) y.sup.2 /2r, .DELTA.X.sub.z is approximately (y.sup.2 /2r)(v.sub.c /v.sub.d). Typical values are a carriage velocity v.sub.c of 0.25 m/sec and a drop velocity v.sub.d of 10 m/sec. For a printhead centered near a platen having a radius r of 0.8 inch, the spot placement from end jets would lag that of the center jets by 0.11 mil for a 1/6 inch printhead, but as much as 1.0 mil for a half inch printhead (assuming all jets were fired simultaneously).
Kuhn et al U.S. Pat. No. 4,158,204 discloses a system for neutralizing errors in printing caused by drop velocity variations from nozzle to nozzle by adjusting the timing sequence which controls the charging of the respective electrodes of each nozzle. Kuhn et al does not compensate for variations in the distance which drops from different nozzles must travel, but only compensates for variations in velocities of the drops expelled by different nozzles due to their differing nozzle characteristics. Kuhn et al does not recognize the problems addressed by the present invention.
Darling et al U.S. Pat. No. 4,167,014 discloses electronic lead determining circuitry that calculates the lead time for projection of ink drops at desired impact positions. The circuitry has detection elements and controlling elements for adjusting to a non-linear movement of the printhead carriage. Darling et al does not compensate for variable distances between different nozzles and the recording medium. Darling et al also does not teach or suggest actuating a column of nozzles sequentially from its ends toward its center.
Yoshino et al U.S. Pat. No. 4,670,761 discloses an ink jet recording apparatus that controls the trajectory of flying ink droplets to adjust to varying relative speed between a rotating drum and a plurality of printheads located adjacent the drum. Yoshino et al does not recognize the problems solved by the present invention and only compensates for variable drum rotation speed, not for drum curvature.
Horike et al U.S. Pat. No. 4,535,339 discloses a deflection control type ink jet recording apparatus in which the velocity of flying charged ink drops is detected and the ink pressure is controlled so as to make the ink velocity coincide with a predetermined target velocity. Horike et al does not teach or suggest the present invention.
Bain et al U.S. Pat. No. 4,524,364 discloses a circuit for use in an ink jet printer in which the carriage motion either approximates a sinusoidal vibratory pattern, or which has any variable velocity pattern that reliably repeats from cycle to cycle. Bain et al does not teach or suggest the present invention.